1. Field of the Invention
The invention relates to a clamping device for a transistor.
2. Discussion of the Background
In high-frequency technology, for reasons of shielding, devices and component groups are generally housed in a shielded housing, that is to say, a metallic housing. Power transistors have a high power loss and must therefore be provided with cooling elements of correspondingly large dimensions.
A corresponding electrical component to be cooled is therefore conventionally connected, for example, to a printed circuit board, in such a manner that its cooling surface is disposed in contact with a cooling element projecting from the printed circuit board and, dependent upon the quantity of heat be removed, provided with a different number of variously-shaped individual ribs. The metal flange of the component to be cooled (high-frequency power transistor) projects through a recess in the printed circuit board and lies with its full surface area on a flat milled recess of the cooling plate. The connecting lugs on the transistor are soldered to the connecting surfaces of the printed circuit board. In this context, the cooling plate serves initially to spread the heat. Dependent upon the design of the cooling channels, which are disposed in the cooling plate, the heat loss can be transferred to a cooling fluid or via cooling ribs, which are in thermal contact with the cooling plate, to the air passing through the ribs. The cooling ribs are typically disposed on the side of the cooling element disposed opposite to the power transistors.
According to DE 43 27 335 A1, a clamping device is provided in order to hold a power transistor in an appropriate position in good contact with the internal surface of the housing wall. A component is held in a predetermined position by means of the clamping device, which consists of a spring clasp or a spring clip. Accordingly, it is possible to use the housing wall itself as a cooling element. The rear support is designed in the form of a bridge engaging over the electrical component, and can be attached by means of screws directly to the housing wall at an adequate lateral spacing distance from the electrical component. The particular disadvantage of this clamping device is the spring clasp acting only in a punctate manner, which does not guarantee a uniform distribution of force to the entire electrical component and therefore achieves only a limited quality of thermal transport to the cooling element.
In the case of DE 43 27 335 A1, mains-voltage-operated components are supposed to be pressed in an electrically-insulating manner onto a cooling plate for the removal of heat. In this context, synthetic material is therefore used as the material for insulating from high electrical voltages and not for interrupting an electromagnetically-functioning transformer winding, which is formed from conductive material around the transistor. One major aspect of DE 43 27 335 A1 is to achieve an assembly through the use of the spring with the relatively-thin housing wall, in order to use the latter as a cooling element at the same time as providing an adequate electrical insulation. This does not come into consideration with the use according to the invention because of the large power loss. In the case of DE 43 27 335 A1, the holding device does not also press on the transistor flange. The pressure here is applied exclusively via the spring.
With regard to the stability of the holding block, DE 43 27 335 A1 is uncritical, because only the component to be cooled is pressed flat against the housing wall. In the case of the exemplary embodiment according to the invention, so much force must be applied by the spring, that the comparatively very-broad transistor flange bends in the center so far that a good thermal and also electrical contact is achieved in the center of the component, from where most of the heat must be removed. In this context, the holding block can preferably absorb this entire force as a counter bearing. The technical feasibility with the use of synthetic material as the material for the holding block is possible only through design measures, so that the latter withstands the forces in the long-term. The large stresses on the holding block are minimized according to the invention in such a manner that the use of synthetic material becomes possible in the first place.
One further difference is evident in the preferred design embodiment: in the case of DE 43 27 335 A1, the threads for the attachment screws are disposed in the holding device. With the exemplary embodiment according to the invention, through-boreholes are disposed in the comparable position. The threads are introduced in the solid cooling plate, because the screws must withstand the pressure of the spring.